A liquid electrographic imaging system includes an imaging substrate onto which a developer liquid is delivered to develop a latent image. A liquid electrographic imaging system may include as the imaging substrate a dielectric substrate, or may take the form of an electrophotographic system having a photoreceptor. In an electrophotographic system, the photoreceptor includes a photoconductive material that is uniformly charged, for example, with a corona charging device. A latent image can be formed on the photoreceptor by selectively discharging the photoreceptor with a pattern of radiation. In an electrographic system that makes use of a dielectric material, the latent image can be formed by selectively charging the dielectric with an electrostatic probe. A liquid electrophotographic imaging system with a photoreceptor will be discussed for purposes of example.
A liquid electrophotographic imaging system generally includes a photoreceptor, an erasure station, a charging station, an exposure station, a development station, an image drying station, and a transfer station. The photoreceptor may take the form of a photoreceptor belt, a photoreceptor drum, or a photoreceptor sheet. For an imaging operation, the photoreceptor is moved past each of the stations in the liquid electrophotographic imaging system.
The erasure station exposes the photoreceptor to erase radiation sufficient to uniformly discharge any electrostatic charge remaining from a previous imaging operation. The charging station electrostatically charges the surface of the photoreceptor. The exposure station selectively discharges the surface of the photoreceptor to form a latent electrostatic image. A multi-color imaging system may include several exposure stations that form a plurality of latent images. Each of the latent images in a multi-color imaging system is representative of one of a plurality of color separation images for an original multi-color image to be reproduced.
As a latent image is formed, the development station applies developer liquid to the photoreceptor to develop the latent image. The developer liquid includes a carrier liquid and developer particles that form colorants. In a multi-color imaging system, each of a plurality of development stations applies an appropriately colored developer liquid to the photoreceptor to form an intermediate representation of the corresponding color separation image. The drying station dries the developer liquid applied by the development station or stations, leaving a film of developer material. The transfer station then transfers the developer material from the photoreceptor to an output substrate, such as a sheet of paper or film, to form a visible representation of the original image.
A development station generally includes a development device such as, for example, a development roller or belt. The operation of a development roller will be described for purposes of example. The development roller is rotated by a drive mechanism and charged with a bias potential that contributes to an electric field between the roller and the photoreceptor. The rotating, charged development roller delivers developer liquid to the surface of an imaging region of the photoreceptor to develop the latent image. The development roller typically is positioned a short distance from the surface of the photoreceptor, enabling a thin layer of developer liquid to be delivered across the resulting gap. In a multi-color imaging system, the development process is repeated with each of a plurality of development rollers applying differently colored developer liquids to the photoreceptor to develop different color separation images.
During operation, back-plated developer can accumulate on the surface of the development roller. The term "back-plated developer" refers to an amount of developer, including developer particles and perhaps a small amount of carrier liquid, that develops on the development roller due to a potential difference between the surface of the photoreceptor and the surface of the development roller. The developer liquid on the rotating development roller wets the surface of the photoreceptor, creating the development nip. When the imaging region of the photoreceptor enters the development nip, the background areas of the image are at an electrostatic potential slightly higher than the development roller bias and the latent image is at an electrostatic potential significantly lower than the development roller bias.
The potential difference between the development roller bias and the latent image results in "forward-plating" of developer to the latent image. The potential difference between the background areas and the development roller bias results in "back-plating" of developer to the surface of the development roller. The back-plated developer retains a small charge that, if allowed to accumulate, will affect the development vector necessary for proper image development. The accumulation of back-plated developer can cause inconsistent transfer of developer liquid to the surface of the photoreceptor. In addition, the back-plated developer can accumulate on other components in the development station, affecting delivery of developer liquid to the development roller.
To avoid excessive accumulation of back-plated developer on the development roller, it ordinarily is desirable to provide an apparatus for removing the back-plated developer. In existing liquid electrographic systems, the developer removal apparatus generally comprises a cleaning blade or cleaning roller. A cleaning blade scrapes developer away from the surface of the development roller. A cleaning roller is rotated to remove the back-plated developer from the development roller. The removed developer is carried away by the surface of the cleaning roller.
The back-plated developer removed from the development roller can accumulate on a cleaning blade or cleaning roller. The back-plated developer has a generally sludge-like consistency and can affect the cleaning efficiency of the cleaning blade or cleaning roller. When the accumulation becomes excessive, the cleaning blade or cleaning roller can actually transfer some of the accumulated developer back to the development roller, undermining the effectiveness of the developer removal apparatus. Excessive accumulation of back-plated developer requires replacement or cleaning of the cleaning blade or cleaning roller by a field service technician.